Digital transmission of signals has become widespread. To this end, a variety of digital transmission media are available which have different transmission characteristics and different information capacity. In order to make efficient use of the various transmission media, a hierarchy of transmission systems has been developed which operates at different transmission bit rates. In North America, the hierarchy includes the DS1 signal transmitted at 1.544 Mb/sec, the DS1C signal transmitted at 3.152 Mb/sec, the DS2 signal transmitted at 6.312 Mb/sec and the DS3 signal transmitted at 44.736 Mb/sec. A similar but different hierarchy is used in Europe.
All of the digital signals to be transmitted enter and leave the digital hierarchy by means of some sort of signal conversion terminal. Additionally, in order to go from one display transmission rate to some other digital transmission rate, one or more multiplexing steps are required. For example, the DS1C transmission signal is obtained by multiplexing two DS1 signals in a M1C multiplexer; the DS2 transmission signal is obtained by multiplexing four DS1 signals in a M12 multiplexer; and the DS3 transmission signal is obtained by multiplexing 28 DS1 signals in a two-step operation of first multiplexing the 7 DS2 signals into the DS3 signal via a M13 multiplexer.
In a so-called MX3 multiplexer, a mix of the DS1, DS1C and DS2 signals are multiplexed into the DS3 format. To this end, four DS1 signals are first multiplexed in a DS1 to DS2 multiplexer to obtain a 6.312 Mb/sec DS2 signal. Similarly, two DS1C signals are each first demultiplexed into two 1.544 Mb/sec signals. Then, the four resulting 1.544 Mb/sec signals are multiplexed in a DS1 to DS2 multiplexer to obtain a 6.312 Mb/sec DS2 signal. The DS2 signal is merely adjusted for system timing purposes to have the MX3 system 6.312 Mb/sec timing. The 6.312 Mb/sec signals are then appropriately multiplexed to obtain the 44.736 Mb/sec DS3 level signal. Thus, multiple multiplexing steps are required and, for the DS1C, the demultiplexing and, then, multiplexing steps are needed. Recovery of the signals from the DS3 format requires similar demultiplexing steps. Such multiple multiplexing and multiple demultiplexing requires additional equipment and, therefore, makes dropping and adding signals at intermediate locations expensive.
For high capacity transmission, it is desirable to be able to easily combine a plurality of one or more of the digital signals in the hierarchy without the need for intermediate multiplexer (demultiplexer) stages or a number of different multiplexing (demultiplexing) schemes. Additionally, it is equally desirable to be able to easily add and/or drop one or more digital signals of one or more digital transmission bit rates without the need of multiple multiplexing and/or demultiplexing processes.